In an imaging apparatus such as a conventional single-unit video-camera recorder (also called "camcoder") or the like, when a gain increasing processing is carried out by a cameraman's operating an operation unit or automatically carried out, a signal processing block for processing an image pickup result changes a gain to process a video signal, which allows an object lacking for illumination to be reliably imaged.
Recently, a picture compression technique for a moving picture is developed. There has been proposed a technique in which a data compression technique according to a moving picture encoding standard such as an MPEG (Moving Picture Experts Group) or the like is applied, data of the video signal can be compressed to about 1/10 in bit rate and the compressed data is recorded on a recording medium such as a video tape, a phase-change magneto-optical disk, a fixed disk or the like, for example.
According to the data compression technique of this kind, difference data obtained by motion compensation between continuous frames or continuous fields is subjected to discrete cosine transform and then quantized. Bits of data used for recording are allocated to the difference data remaining after the motion compensation. Thus, the video data is efficiently recorded by effectively utilizing correlation between the video data of this kind. There has been considered a method of effectively recording the image pickup result in the form of the digital signal by applying such data compression technique to the single-unit video-camera recorder.
When such data compression technique is applied to the imaging apparatus such as the single-unit video-camera recorder, bits are intensively allocated to a random noise after a gain increasing processing, and consequently a picture quality is deteriorated to that extent.
Specifically, in such data compression technique, a motion compensation processing eliminates components which are common in the continuous frames and residual difference data is encoded, thereby data being efficiently stored or transmitted. On the other hand, a noise is generally included at random in a video signal obtained by the imaging apparatus such as the single-unit video-camera recorder. Since such random noise does not provide correlation between the continuous frames, such noise is recognized as "a motion" and hence a large number of bits are allocated to the noise, i.e., bits are intensively allocated thereto.
Further, if such video signal is subjected to the gain increasing processing, the noise is disadvantageously increased to that extent. Especially, when carrying out .gamma. correction, such imaging apparatus outputs the image pickup result whose low luminance part includes an intensive noise. Since the gain increasing processing is frequently carried out when a dark object is imaged, the random noise of the low luminance part is remarkably increased.
Therefore, if the image pickup result whose gain is increased is motion-compensated, then signal components of the random noise included in the difference data are increased to that extent. Consequently, bits of the recording data are intensively allocated to the random noise. As a result, it is observed not only that increase of the random noise resulting from the gain increasing processing deteriorates the picture quality but also that necessary information between frames cannot be transmitted, which remarkably deteriorates the picture quality processing of the recorded digital signal to that extent. Specifically, a pseudo contour or the like is likely to be generated in a picture portion.
While a possible method of solving this problem is a method of eliminating such random noise by applying a noise reduction system, application of the noise reduction system still leaves the problem of resolution deteriorated when the gain increasing processing is not subjected.
In view of the above problems, it is an object of the present invention to propose an imaging apparatus which, even when a gain of a video signal is changed, can prevent bits for an image pickup result from being intensively allocated to a noise.
Moreover, it is another object of the present invention to propose an imaging apparatus which, when a gain is changed, can reduce a noise in proportion to change of the gain.